Passive infrared (PIR) detectors are coming into use to control the switching of ac mains. The PIR detector can have its heat sensor mounted on the face plate of a standard electrical connection box containing the electric relay switch that selectively connects the ac power through the box under control of the heat sensor. The electric relay switch may be of electromechanical type or of semiconductor switching device type, by way of examples. The small electric power output of the heat sensor is customarily amplified to obtain enough power to control the electric relay switch. The dc power supply for this amplifier is difficult to provide for when ac common is not routed through the box, since there is no alternating potential difference readily available from which to generate direct potential.